A - Means tight Match(will small 0.05mm than standard)B - Means loose Match(will larger 0.05mm than Standard) Diameter: (Teeth +2) * modulus = Diameter of Modulus: 0.5Aperture 1.5/2/2.5/3Spindle (9 styles): 8-1.5A 9-2A 8-2A 10-2A 12-2A 14-2A 16-2A 18-2.5A 18-3ACrown tooth(10 styles): C20-2.5A C20-2A C20-3A C2410-2A C2410-2B C2810-2B C28-2A C3010-2B C30-3A C3610-2BSingle (18 styles): 20-2A 26-2A 24-2.5A 28-2A 30-2A 36-2A 38-2A 38-3A 40-2A 42-2A 44-2A 44-2.5A 46-2.5A 48-2A 50-3A 52-2.5A 56-2A 56-3ADouble (18 styles): 1810-2A 4812-2.5A 2210-2B 2410-2B 2610-2B 2808-2B 4610-2B 2810-2B 3010-2B 3210-2B 3212-2B 3412-2B 3610-2B 3808-2B 4410-2B 4810-2B 5010-2B 5610-2AWorm (2 styles): 6 * 6-2A 6 * 8-2APulley (1 styles): 6*6-2APackage Contents:58 styles Plastic Gears All The Module 0.5 x 1 lotsOnly the above package content, other products are not included.Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.